『Abstract
In the greenhouse world of the Cretaceous, there were episodes
of oxygen depletion in the deep ocean associated with enhanced
organic carbon burial in sediments (black shale formation). In
this study, we use a box model of the oceanic phosphorus (P),
organic carbon (orgC) and oxygen cycles to explore the hypothesis
that variations in marine P availability control deep ocean oxygen
depletion (anoxia) and the formation of black shales under Cretaceous
ocean conditions. We find that, for the Cretaceous ocean, with
large continental shelves, slow oceanic overturning and high sea
surface temperatures, Oceanic Anoxic Events (OAEs) can be triggered
by enhanced P supply from land, and that the system is particularly
sensitive to oceanic mixing. In our baseline scenario, the deep
sea becomes completely anoxic, while the shelves attain only partial
anoxia. Sedimentary burial differs between the shelves and open
ocean: while organic carbon burial is enhanced in both regions,
deep sea reactive P burial decreases dramatically under anoxia,
but not on the shelves, where oxygen depletion is not complete.
Furthermore, our model results imply that OAEs can be sustained
by P recycling from sediments under low-oxygen conditions. Ultimately,
however, the feedbacks which result in the accumulation of atmospheric
oxygen terminate the anoxic event. Atmospheric oxygen is modulated
by land process such as forest fires and oxidative weathering,
which limit the rise of atmospheric O2. Our
model findings are corroborated by P burial data from the geological
record for OAE2 (approximately 94 Myr BP). Through a sensitivity
analysis we identify two necessary criteria for OAEs: low mixing
of surface and deep waters (poor ocean ventilation) and enhanced
sedimentary P recycling under low oxygen conditions. When these
criteria are met, ocean anoxia is a robust result to a mild increase
of continental supply of phosphorus, under a wide range of environmental
conditions.
Keywords: phosphorus; Cretaceous; Oceanic Anoxic Event; marine
black shale; modeling; DSDP Site 530; ODP Site 1260』
1. Introduction
2. Model description
3. Results and discussion
3.1. General OAE scenario
3.2. Redox-dependent P burial
3.3. Effects of ocean conditions and external forcings (shelf
size, mixing, temperature and weathering)
4. Conclusions and implications
Acknowledgements
References